Several thousand snow geese may have died after landing in a former open-pit copper mine in Montana that is now filled with highly acidic water.

A Nov. 28 snowstorm forced the migrating geese to take refuge in the Berkeley Pit, a 900-foot-deep pit lake of heavy metals and other dangerous chemicals that is now a federal cleanup site in Butte, Mont.

The site has seen a horror like this before: In 1995, 342 geese landed on the pit, drank its water, and suffered fatal burns to their tracheae and other internal organs.

Butte once had a more glamorous reputation — “The Richest Hill on Earth.”

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How did it become home to one of the nation’s most infamous Superfund sites, a place still capable of killing thousands of birds long after it ceased operations as a mine?

At the time the Berkeley Pit began operating in 1955 under the ownership of the famous Anaconda Co., copper prices were at their highest since the end of World War II. They spiked again in 1974, during the Vietnam War, and the mine eventually produced up to 1 billion tons of copper ore and waste rock. But prices steeply declined after that, leading the administrators of the mine to stop operations in 1982.

Slowly, the 1,780-foot pit began filling with groundwater. The water reacted with sulfides and metals in the soil, gradually creating a body of water that was less like a lake than it was a vat of poisonous stew.

One of the chief concerns is that the pit water level will eventually reach the level of the natural water table. The former mine could then pollute nearby Silver Bow Creek and the headwaters of the Clark Fork River. Projections show that could happen by about 2020.

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The federal Environmental Protection Agency, which has been managing the cleanup as a federal Superfund site since 1983, is making a lot of progress in removing waste from the site, says Joe Vranka, the EPA’s supervisor of Montana Superfund sites.

But EPA officials are allowing the basin to fill to a critical level before requiring Montana Resources and Atlantic Richfield Co., the current managers of the mine site, to begin pumping and treating the water to prevent contamination of nearby waters, Vranka said.

Until that point, the toxic lake remains open to any snow geese and other birds that might land there — exactly what happened in 1995 and again over the last week.

After the 1995 incident, mine managers put in place ways to haze birds that attempt to land near the mine, including using spotlights, shotgun and rifle noises and electronic devices that emit predator noises to scare birds away.

Those efforts largely seemed to work in recent years, with only 14 snow geese reported killed from 2010 to 2014.

But last month the system failed, said Janet Ellis, senior director of policy for Montana Audubon. An unusually large number of geese were migrating south at a much later time than usual, and they were also trying to beat a south-moving storm.

“They ended up being a lot more rushed, at the same time trying to make more miles quickly, so they landed at the Berkeley Pit,” Ellis said.

The usual deterrents weren’t enough, she said.

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“One of the challenges is that noise is not the best deterrent for wildlife. Different animals react differently, and they can get used to it,” Ellis said.

Another challenge, she said, is that birds are attracted to water where other birds are, and in this case, the number of birds that landed on the pit could have completely overwhelmed the deterrence system.

Mark Thompson, environmental affairs manager for Montana Resources, said employees tried urgently to drive the birds away and are continuing to keep birds from landing on the pit.

The EPA could issue Montana Resources and Atlantic Richfield Co. fines if it deems the companies were not in compliance with the hazing program.

Snow geese are protected under the Migratory Bird Treaty Act, which says the birds can’t be killed other than with a permit from the U.S. Fish and Wildlife Service.

Nina Agrawal is a staff writer for the Los Angeles Times. She previously reported for WLRN-Miami Herald News and for the Latin American affairs magazine Americas Quarterly. A Southern California native, Agrawal is a graduate of the University of Pennsylvania and Columbia University’s Graduate School of Journalism and School of International and Public Affairs.